Stars emit electromagnetic radiation. Within that spectrum are gamma rays, X-rays, radio waves, microwaves, infrared light, visible light and ultraviolet light. The Earth's atmosphere blocks most rays that are harmful to life, but without some of that energy, life on our planet would not exist. Plants need light to make food energy. Animals eat those plants, or plant-eating animals, and thereby derive that food energy for themselves. Within the radiation that reaches Earth, blue light plays a key role in food production.
What humans perceive as visible light occupies a sliver of the electromagnetic spectrum, with wavelengths between 375 and 775 nanometers. Of that range, blue light measures 420 to 440 nanometers, on the shorter side of the lengths of visible light rays. This shorter wavelength is readily absorbed and refracted by water vapor molecules in the atmosphere, which is why the sky appears blue. Thus, blue light is readily available to outdoor plants.
How Plants Use Blue Light
Blue light is the most critical frequency range of the visible light spectrum for plants. A photoreceptive molecule called chlorophyll absorbs photons from blue light and uses that energy to drive photosynthesis. This reaction converts carbon dioxide and water into glucose and oxygen. The plant stores the glucose for use as food, and discards the oxygen as waste.
How Blue Light Affects Plants
Plants with access to abundant blue light, assuming every other nutritional need is met, will be lush and green, with much foliage. They will grow relatively slowly, sprouting many new leaves as the stems lengthen. If only blue light is supplied, these lush plants may never bloom, as a certain quantity of red light is need to trigger hormonal changes that build reproductive structures.
Without Blue Light
Without sufficient blue wavelengths of light, plants look long, thin and scrawny because what little energy they have is used to grow a long stem in search of a good light source. As time passes, these plants will lose color as the green-shaded chlorophyll is cannibalized by the plant for use in other cell functions. Eventually, the plant will die.
Garden outlets sell "grow lights" that provide those wavelengths of the light spectrum that benefit plants most, but they can be expensive. "Cool white" fluorescent bulbs, common in kitchens and bathrooms, emit abundant blue light. If placed closely enough, indoor plants with no other light source will thrive. Blooms may be scarce due to the few red light rays these bulbs produce, but the plants themselves will be green and leafy.